1. Field of the Invention
The present invention relates to an optical pickup device using a diffraction element such as a holographic optical element and an optical recording medium driving apparatus comprising the same.
2. Description of the Background Art
In recent years, an optical pickup device using a holographic optical element has been studied and developed with demands for the miniaturization and the light weight as well as the low cost of the optical pickup device. This type of optical pickup device is disclosed in Japanese Patent Laid-Open No. 124205/1996, for example. FIG. 35 is a side view showing the schematic construction of a conventional optical pickup device, and FIG. 36 is a plan view showing the schematic construction of the conventional optical pickup device.
In FIGS. 35 and 36, in the conventional optical pickup device, an electrically conducting heat sink 3, a semiconductor laser device 5, a transmission type three-beam generating diffraction grating 6, a transmission type holographic optical element 7, and a reflecting mirror 8 are arranged on a main surface of a substrate 2.
The substrate 2 is made of an electrically conducting semiconductor material such as n-type Si (silicon), a good thermal conductive material composed of an electrically conducting metal such as copper, resin, or the like.
The semiconductor laser device 5 formed on an n-type Si semiconductor substrate is put on the electrically conducting heat sink 3. A pin photodiode 4a is formed on a main surface of the electrically conducting heat sink 3, and a photodiode 4b for information signal detection is formed beside the photodiode 4a. Further, the semiconductor laser device 5 is mounted on the main surface of the electrically conducting heat sink 3. The transmission type three-beam generating diffraction grating 6 is fixed to a groove 2a of the substrate 2 so as to be positioned ahead of a light emitting portion of the semiconductor laser device 5. The transmission type three-beam generating diffraction grating 6 has a diffraction grating surface 6a composed of concavities and convexities at an equal pitch on its surface on the side of the semiconductor laser device 5, and divides laser light emitted from the semiconductor laser device 5 into zero-order, +1st-order and -1st-order diffracted light beams and emits the light beams.
The transmission type holographic optical element 7 is fixed to a groove 2b of the substrate 2 so as to be opposite to the transmission type three-beam generating diffraction grating 6 on the light emission side of the transmission type three-beam generating diffraction grating 6. The transmission type holographic optical element 7 comprises a transparent substrate having a holographic functional surface 7a composed of a group of curves whose concavities and convexities are at a gradually changed period formed on its surface on the side of the transmission type three-beam generating diffraction grating 6.
The reflecting mirror 8 is fixed to a groove 2c of the substrate 2 so as to be inclined through an angle of 45.degree. with the transmission type holographic optical element 7 on the light emission side of the transmission type holographic optical element 7. The reflecting mirror 8 reflects the three diffracted light beams passing through the transmission type holographic optical element 7 upward at approximately right angles.
An objective lens 9 is arranged above the reflecting mirror 8, and focuses the diffracted light beams reflected by the reflecting mirror 8 on a recording surface of a reflection type optical recording medium 1, to form a main spot caused by the zero-order diffracted light beam and two sub-spots caused by the .+-.1st-order diffracted light beams on both sides of the main spot.
Furthermore, a reflecting mirror 10 focuses into the photodiode 4b the three returned light beams from the optical recording medium 1 which include information signals in the main beam and the two sub-beams according to the main spot and the two sub-spots respectively.
In the above-mentioned optical pickup device, the laser light emitted from a rear facet of the semiconductor laser device 5 is received by the photodiode 4a. The photodiode 4a outputs a signal corresponding to the amount of received output power of the laser light. An automatic power control circuit (not shown) controls the semiconductor laser device 5 such that light output power of the laser light from the semiconductor laser device 5 is constant on the basis of the signal from the photodiode 4a.
On the other hand, the laser light emitted from a front facet of the semiconductor laser device 5 is divided into three zero-order and .+-.1st-order diffracted light beams by the transmission type three-beam generating diffraction grating 6, after which the three diffracted light beams are incident on the transmission type holographic optical element 7. The three diffracted light beams passing through the transmission type holographic optical element 7 are reflected upward by the reflecting mirror 8, and are then focused as a main spot and two sub-spots by the optical recording medium 1 using the light-focusing function of the objective lens 9. The three diffracted light beams focused as the main spot and the two sub-spots on the optical recording medium 1 are reflected as three returned light beams including the information recorded on the optical recording medium 1 on the surface of the optical recording medium 1. The three returned light beams pass through the objective lens 9, is reflected by the reflecting mirror 8, and is then incident on the transmission type holographic optical element 7.
The three returned light beams passing through the transmission type holographic optical element 7 by 1st-order (or -1st-order) diffraction pass above the diffraction grating surface 6a of the transmission type three-beam generating diffraction grating 6, are then reflected downward by the reflecting mirror 10, and are incident on the photodiode 4b.
A reproducing signal, a focus error signal produced by a well-known astigmatism method, and a tracking error signal caused by a well-known three-beam method are obtained on the basis of the returned light beams incident on the photodiode 4b. Consequently, playback of the information recorded on the optical recording medium 1, tracking servo and focusing servo are performed.
In the optical pickup device, the photodiode 4b for returned light detection and the semiconductor laser device 5 are provided on the common main surface of the electrically conducting heat sink 3. Therefore, a lead frame member electrically connected to the photodiode 4b and a lead frame member electrically connected to the semiconductor laser device 5 are flush with each other. Therefore, the width of a case of the optical pickup device is increased, so that the optical pickup device is increased in size.
In the above-mentioned optical pickup device, the reflecting mirror 10 is movably and rotatably mounted in order that the returned light beams reflected by the optical recording medium 1 are incident on the photodiode 4b in their most suitable states. Therefore, a mounting mechanism of the reflecting mirror 10 is complicated and is increased in size, so that the thickness of the case of the optical pickup device is increased.
Furthermore, the fabricating steps of the optical pickup device comprise the output inspecting step of the semiconductor laser device 5. FIG. 37 is an explanatory view of the step of inspecting the semiconductor laser device in the conventional optical pickup device shown in FIGS. 35 and 36. The inspecting step is carried out in a state where the electrically conducting heat sink 3 and the semiconductor laser device 5 are arranged on the main surface of the substrate 2, laser light B is emitted from the semiconductor laser device 5, and the light intensity distribution of the laser light B is detected ahead of the semiconductor laser device 5. The light intensity distribution of the laser light B is referred to as a far-field pattern FFP. The divergence angle and the shift in position of the optical axis of the laser light B from the semiconductor laser device 5 are detected by measuring the half-width W and the peak position P of the light intensity on the basis of the far-field pattern FFP.
In the above-mentioned optical pickup device, however, the laser light from the semiconductor laser device 5 is emitted approximately parallel to the main surface of the substrate 2, and is radially enlarged as it travels. Therefore, a part of the laser light B enlarged toward the main surface of the substrate 2 is prevented from directly traveling upon striking the main surface of the substrate 2, so that a missing portion L occurs in the far-field pattern FFP. Therefore, an accurate far-field pattern FFP is not obtained, so that errors occur in the spreading inspection and the inspection of the shift in position of the laser light from the semiconductor laser device 5.